1. Field of the Invention
The present invention relates to a data storage system having a plurality of external storage units, such as magnetic disc units, magnetic tape units and so on, and an interface bus commonly connecting the external storage units to a host computer.
2. Description of the Related Art
Recent developments in computer systems have led to demands for a higher reliability of such systems, and accordingly, it has becomes important to increase the reliability of peripheral equipment for a computer, e.g., an external storage unit such as a magnetic disc unit, magnetic tape unit and the like.
To meet this demand, a variety of external storage units having an increased reliability have been proposed. However, in view of recent trends toward a continuous operating of the external storage units, i.e. day and night without stopping the system, the external storage units operate under very severe circumstances. Therefore, there is an increasing incindence of the occurrence of problems with the related units, and thus faults inevitably occur in the system.
To prevent the occurrence of such faults, and to perform necessary maintenance and inspection, a running external storage unit must be replaced with a new unit. In this case, the related system must be disconnected as a whole from the power source to protect data that may be lost by the power disconnection. However, power disconnection, is contrary to the requirements of the above-mentioned continuous operation of the system.
Accordingly, there is a need for a way to replace the external storage unit without disconnecting the power from the system, to thereby allow continuous and operation of the system.
Currently, as mentioned above, when any one of the external storage units, such as the magnetic disc units, the magnetic tape units and so on, associated with a computer system, is to be replaced with a new unit, for maintenance and inspection of the unit, it is necessary to disconnect the power source from the system, as a whole, before commencing the related maintenance and so on.
Practically, in a fault-tolerant computer system having nonstop operation, day and night, the system is very reluctant to spare or spend time for the replacement of the external storage units, because the spared time is equivalent to a wasteful waiting time for a user of the computer system. Thus, it is not preferable to be able to easily disconnect the system from the power source in view of maintaining the reliability of the nonstop operation.
Currently, the system power must be disconnected, to replace the external storage units, which is typically done at night or at a time when the computer system is not processing a heavy load. This, however, limits the opportunities of achieving the above-mentioned maintenance and so on.